Trouble With Paint Blistering of Paint Films on Metal Part 1 Osmotic Blistering

TROUBLE with PAINT

Blistering of Paint Films on Metal, Part 1: Osmotic Blistering

his month’s column will dis-cuss the practical effects ofwater absorption into applied

coating systems on metal. The articlebegins a two-unit review of blister-ing failures and delaminations interms of the driving forces that pro-duce them. At this stage, our discus-sion will only address blistering phe-nomena on metal. While blisteringcan affect the service of coatings onconcrete and other substrates, thesewill be considered in later segmentsof this series, when we direct specif-ic attention to the substrates. Blister-ing or bubbling produced by agentsother than water, such as gas- orsolvent-induced blistering, will notbe considered.

Because metal is impervious towater, water cannot access the inter-face through the substrate (from be-neath), as may occur on coatedwood and concrete. It must always

pinholes, abrasions, and holidays.An important key to understanding

first go through the coating or atleast progress along the interfacefrom locally exposed sites such as

JPCL – PMC / FEBRUARY 1998 45

T

continued

Fig. 2 - Mechanism of osmotic blistering of coating films

Fig. 3 - Sources, sites, and effects of osmotic blistering in coating films

by Clive H. Hare, Coating System Design Inc.

Fig. 1 - Osmotic blistering of coating on steelFigures courtesy of the author

Low solute conc.

High solute conc.

Low solute conc.

High solute conc.

Low solute conc.

High solute conc.

Low solute conc.

High solute conc.

Indicates location of failure

Water

Paint film [semi-permeable membrane]

Water-soluble speciesSubstrate

Water from environment is absorbed by film. At lower interface, it contacts soluble species beneath film.

Water dissolves the soluble species, forming a concentrated solution of low osmotic pressure.

As solution concentration drops with additional migration of water, osmotic pressure becomes too great for the adhesive forces holding paint film to the substrate and results in the localizeddelamination of film as a solution-filled blister.Water continues to be drawn through the film until the osmotic pressure on opposite sides of the membrane equilibrates.

While complete equilibrium is never achieved as the osmotic differential decreases, so the rate of water migration and the rate at which the blister size increases also grow less.

Aqueous environment

Finish coat

Primer

Steel substrate

Solublespecies

Blistering, delamination, substrate deterioration

Blistering, delamination, corrosion

Blistering, reduced cohesion, film splitting, delamination

Blistering, reduced cohesion, delamination, corrosion

Blistering, intercoat delamination

From components of paint film

(e.g., inhibitive pigments, solvents,

additives, etc.)

From components

of lower paint films(e.g., inhibitive

pigments, solvents,

additives, etc.)

From contamination of lower paint

films(e.g., salts)

From substrate

(e.g., metallic corrosion product)

From contamination

of substrate(e.g., salts)

Copyright ©1998, Technology Publishing Company

Copyright ©1998, Technology Publishing Company JPCL – PMC / FEBRUARY 1998 47

TROUBLE with PAINT

the cause of blistering on metal canbe found by isolating the drivingforces that ensure the unidirectional-ity of water flow through the film tothe substrate and there sustain theconsequent accumulation.

Several recognized driving forcesare associated with the productionof blistering in coatings on metal bydiverse mechanisms. These includeosmotic gradients, producing blister-ing under fresh water conditions;electroendosmotic gradients, pro-ducing blistering in ionic solutions;and thermal gradients, producingthe cold wall blistering often seen inhumid environments. Cathodic blis-tering, produced by the generationof alkalinity at the cathode, is alsoassociated with electrical gradientsand is often driven by an externallyimpressed current. It is commonly aconsequence of the application ofhigh potential differences acrosscoated substrates.

In this article, we will consideronly osmotic blistering, which isthought to be the most prevalenttype of failure. The article will dis-cuss the mechanism of osmotic blis-tering, the factors contributing to os-motic blistering (including thenature and source of the solute),and the sources of osmotic gradientsfrom the corrosion process, retainedsolvents, and non-carrier solvents.Corrosion caused by osmotic blister-ing will be characterized, and os-motic blistering at pinholes will be described.

The Mechanism of Osmotic BlisteringWe noted in the November 1997column that intact paint films aresemi-permeable membranes, perme-able to water, but impermeable todissolved solids. This model is pre-cisely that which accommodates os-motic blistering. The phenomenon

(Fig. 1) depends on the presence ofa water-soluble material at either theinterface of the paint film with thesubstrate, or, in multi-coat systems,at some intermediate interface that iscovered by another coat of paint.Often, the active material is an inor-ganic salt of some kind. In addition,the external face of the paint film(or system) must be in contact withan aqueous environment that is ei-ther free of or lower in dissolvedmaterial than the environment be-neath the film.

Under such conditions, after wateris absorbed by the film, it is subse-quently transferred to the lower filminterface (e.g., metal substrate).There it may come in contact withthe soluble material on the substrateand leave the film to dissolve thematerial. Under fresh water condi-tions (distilled water or even highhumidity), such sub-film dissolution

continued

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TROUBLE with PAINT

creates a concentration gradientacross the film, which here acts as asemi-permeable membrane. On thedownstream side of the film wherethe solute is dissolved by water fromthe film, the solute concentration ismuch higher than is the solute con-centration at the external (or up-stream) face of the film. Under theseconditions, water will be drawnthrough the film towards the con-centrated solute, under osmoticpressure. This transfer of water oc-curs because the water pressure andsalt concentrations on either side ofthe membrane attempt to equili-brate. The mechanism of osmoticblistering is illustrated in Fig. 2.

In quantitative studies of the phe-nomenon, van der Meek-Lerk andHeetjes1 have shown that blistersinitially grow fast, but the growthslows with time. Growth is still mea-surable after 160 days’ immersion.Accompanying this growth is a pro-gressive decrease in salt concentra-tion within the blister, which in-creases the water concentration andprogressively reduces the drivingforce of the growth.

External water that is relativelyhigh in dissolved salts (e.g., saltwater) will not favor the formationof osmotic gradients. In sea water,osmotic blistering is not normally aserious problem. However, thedeionized water and condensatefound in power generation facilitiesas well as potable water found intanks and stand pipes may be partic-ularly troubling. Rain water and high humidities are more general-ized sources of water of low soluteconcentration, which can result inhigh osmotic pressure. But the dura-tion of the “immersion” in simpleexterior environments is normallytoo short to produce problems.However, persistent humidity andcondensation in such environmentshas caused problems with filmsbased on highly soluble corrosioninhibitors, such as zinc yellow.

The Nature and Sources of the SoluteInorganic SaltsThe nature of the solute below thesemi-permeable membrane seemsunimportant.1 Osmotic blistering hasbeen related not only to chlorides,sulfates, and other inorganic sol-ubles often found on substrates, butalso to organics such as sugar.

Notwithstanding this, blisteringfrom aggressive depassivating saltssuch as chlorides and sulfates are ofparticular concern to the protectivecoatings engineer. These materials(unlike rust itself) readily acceleratefurther underfilm corrosion and blis-tering. Regarding corrosion, there isfar more evidence of critical thresh-olds necessary to its initiation thanthere seems with respect to blister-ing. Estimates of permissible salt lev-els for underfilm rusting vary from1.2 mg Cl-/cm2 and 10 mg SO4

=/cm2 (Igetoft2) to 500 mg Cl-/m2

(West3) and 50-100 mg SO4= (Mor-

cillo4). The subject is reviewed indetail by Alblas and van Londen.5

In the author’s opinion, the searchfor permissible salt concentrationthresholds at which corrosion willnot occur is inevitably complicatedby the variety of individual modelspossible. Not only is the relationshipcomplicated by film thickness, butinfinitely more so by film character-istics and the mechanism of corro-sion control. (Zinc-based systemsare far less vulnerable to salt conta-mination than are barrier systems,for example.) Inhibitor-based sys-tems, relying on anodic passivationcontrol, will be particularly vulnera-ble to these contaminations. Tolera-ble levels will depend upon the typeand loadings of inhibitor used, pig-ment volume concentration (PVC)/critical pigment volume concentra-tion (CPVC) ratio, pH of the mi-

continued


croenvironment beneath the coat-ings, and temperature. Other charac-teristics of the binder itself—perme-ability to water, oxygen, saponificationresistance, and dielectric constant—will all have an effect. Thus, itwould appear that permissible saltlevels for underfilm corrosion resis-tance, if not good blistering resis-tance, will safely be the lowest levelderived from the general experience,unless the thresholds for the particu-lar system are known. Unfortunately,in many models, this position willinevitably lead to over-engineering.

For osmotic blistering alone, thetype and molar concentration ofsolute seems most important to thesize and morphology of the blisterlevel. Morcillo et al6 found thatwhile ferrous sulfate concentrationsproduced a large number of fineblisters, sodium chloride inducedfewer but larger blisters. The actualdifference in the solute concentra-

tion on either side of the paint filmmembrane need not be large to sup-port the continued growth of theblister. As is noted by van der Meek-Lerk and Heetjes, even traceamounts of hydrophilic surface cont-aminants may be sufficient to causeosmotic blistering.1

Blistering patterns reminiscent offingerprints have betrayed untowardhandling practices and the transferof perspiration onto the steel byworkers before painting. Most typi-cally, it is, however, airborne saltssuch as chlorides derived from ma-rine environments, bridge deicingsalts, sulfates produced by acid rainand industrial effluent (SO2, SO3),and nitrogen oxides that cause fre-quent trouble. Abrasives (especiallysilica sand) have also been noted asa source of salt contamination7, al-though SSPC Report 91-07 showedthat the amount of salt transferred tosubstrates from abrasive was ex-

tremely low and did not produceblistering under conditions ob-served.8 The number and variety ofinorganic contaminants found onbridge structure surfaces is surpris-ingly large.9

Significantly, solubles accidentallyor deliberately entrained in the coat-ing itself may also cause difficulties.These materials may be readilytransferred to the interface in waterservice. Highly soluble inhibitivepigments, such as chromates, molyb-dates, and borates within primerfilms may cause osmotic blisteringeither between coats or at the metalinterface beneath the primer (Fig. 3).In 1991, the Pittsburgh Society forPaint Technology found that blister-ing could be related to the amountof water-extractable material in thepaint film.10

Similarly, soluble species may bederived as a result of reaction or

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50 FEBRUARY 1998 / JPCL – PMC

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Copyright ©1998, Technology Publishing Company JPCL – PMC / FEBRUARY 1998 53

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degradation of pigment or binder.Investigating severe blistering of analkyd film in a mildly alkaline envi-ronment (pH 8.2), Bullet and Prosserattributed the phenomenon to a sol-uble residue derived from the hy-drolysis of the binder.11

Elm12 reports instances of osmoticblistering over zinc-rich primers thatmay become contaminated withwater-soluble salts after priming. Re-dress may be difficult because of theporous nature of many zinc-richfilms, especially in cases where thezinc film binder is non-soluble insolvents of the finish coat. Washingand rinsing the primer thoroughlywith fresh water before topcoatingmay lessen if not eliminate the prob-lem. Before recoating, the newlycleaned primer must be dry.

The Products of the Corrosion ReactionOsmotic gradients may also arise di-rectly from the corrosion process it-self. Water and oxygen can directlyaccess the metal at isolated sites ofnon-adherent film and crevices be-neath the film where the paint failedto wet the substrate. Their corrosionproducts, which have some solubili-ty in water, may cause trouble. Anexample of these corrosion productsis Fe(OH)2. These too may initiateosmosis, especially where oxygenconcentrations are low enough todelay the secondary oxidationprocess to the more insoluble ferric(Fe(III)) compounds. Where chlo-rides and sulfates are simultaneouslypresent, the corrosion products (fer-rous chloride hydrate and ferroushydroxy sulfate) are even more solu-ble in water. With certain vulnerableester-based binders (e.g., oil alkyds),the high pH generated at the cath-ode may hydrolyze the polymer.This reaction not only loosens theadhesion of the coating at the pe-riphery of a blister (so enlarging it),but also simultaneously producessoluble degradation products such

as alkali metal soaps, which also en-courage osmotic blistering.11

Retained SolventsOsmotic blistering can also becaused by the retention of hy-drophilic solvents and diluents with-in the film (most often high boilingalcohols, glycol ethers, and esters).Under suitable conditions, these sol-vents may remain in the film formonths. Many of these solvents are

highly miscible with water andresidual quantities of these materialswill draw water through the film os-motically, similar to the effects ofsoluble salts. Blistering similar in ap-pearance to blisters caused by sub-film salts will result from solvent re-tention. The reasons for suchdiscrete blister formation in filmshaving supposedly uniform solventdistribution (as opposed to the

continued

Fig. 5 - Corrosion process in osmotic blister

Fig. 4 - Phase separation, microvoiding, and solvent entrapment as a cause of osmotic blistering

AS SOLUBILITY OF BINDER IN SOLVENT SYSTEM IS PROGRESSIVELY IMPROVED, PHASE SEPARATION AND MICROVOIDING OCCUR LATER AND LATER IN THE FILM FORMATION PROCESS.

Phase separation (resin precipitation) from non-solvent systems occurs very early during film formation. Film cohesion is poor with open pores.

Phase separation from increasingly better solvent system occurs later during film formation, and microvoiding occurs progressively closer to interface.

No phase separation occurs in films deposited from good solvent system. No microvoiding occurs, even near interface.

Residual hydrophilic solvents will occupy microvoids in lower layers of film and attract water into film, setting up osmotic blistering.

Phase separation from poor solvent system occurs early during film formation.

a.)

b.)

c.)

d.)

Oxygen-rich water is drawn to interface osmotically, accumulating in layers until film delaminates.

Iron dissolves, going into solution as ferrous ions and, in the presence of oxygen, forming soluble ferrous corrosion product.

Cathodic hydroxyl at periphery of blister causes film to delaminate, so that blister expands, rust forms, and cathode sites advance outward.

Fe++ corrosion products are rapidly oxidized toFe+++ products, which are deposited as rustlayer on underside of blister dome, cutting offoxygen supply to blister interior.

At periphery of blister, oxygen availability through paint film is higher, allowing cathode reaction.

Ferrous corrosion product

Rust layer


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wholesale delamination of films soaffected) is explained by Funke.13

Funke has investigated the mor-phological structure of paint filmsdeposited from mixtures of solventsand diluents.13,14 This research hasshown that the onset of incompati-bility and phase separation in filmscontaining low boiling solvents andhigh boiling diluents will dependupon 3 factors:• the type and ratio of the solventsand the diluents; • the application temperature; and • the glass transition temperature(Tg) of the binder.

The morphological structure of thefilm depends on when incompatibil-ity sets in during conversion of thefilm from liquid to solid. In well-for-mulated systems, films pass into theglassy phase without any phase sep-aration at all, and clear, continuousfilms result. In systems with higherconcentrations of high boiling dilu-ents, incompatibility and phase sep-aration set in more rapidly, poten-tially resulting in various anomalousmorphologies. In extreme cases, thedevelopment of incompatibility earlyin the film formation process willproduce precipitation of the binder,resulting in a non-continuous film. Ifphase separation occurs only slightlybefore the onset of gelation, the filmmay be coherent, but it will tend tocontain large, open pores. Phaseseparation occurring subsequent togelation will result in a closed, con-tinuous surface layer covering a mi-crovoided interior. As phase separa-tion occurs nearer and nearer to thetime that the film passes into the Tgstate, the film will be progressivelyfreer of microvoids.

Significant to our discussion of os-motic blisters is the fact that the re-duced microvoiding, noted as phaseseparation that occurs later and laterduring film formation (or as the sol-vent system progressively improves),does not uniformly disappear.Rather, it shifts downwards, occur-

ring at deeper layers of the filmnearer to the interface, at which lo-cation it is most likely to be found.Hydrophilic diluents and marginalsolvents are found primarily withinthese microvoided areas close to theinterface (Fig. 4).

Under conditions favoring osmo-sis, water diffuses through the filmtowards these microcellular inclu-sions adjacent to the interface. Therate of water diffusion under osmot-ic pressure differentials is muchgreater than is any tendency of theentrapped solvents and diluents todesorb water from the film. Thus,water accumulation is progressiveand results in a blister pattern re-sembling the microstructure itself.Funke14 used vinyl lacquers for hisinvestigation. In practical protectivecoating systems, the formulationpractices (low boiling solvent, highboiling diluents) he employed arethe exception rather than the rule.

However, with many thermosets,Funke’s arguments become morevalid. Their molecular weight in-creases and solubility profileschange as a result of some conver-sion process (chemical cure and oxi-dation). Some degree of phase sepa-ration may even occur with truersolvent systems as the cure progress-es and solvency decreases.

These phenomena are aggravatedby increasing film thickness, wherethe solvent (and non-solvent) reten-tion is greater and the microcellularstructure more entrenched. The blis-ters are often found to contain waterand hydrophilic solvent (diluent), al-though corrosion may not immedi-ately initiate.

If the film is post baked at temper-atures well above the Tg before ex-posure, the offending solvent maybe released, and the osmotic pres-sures will not develop.

continued


Osmotic Blistering by Non-carrier Solvents from theService EnvironmentRelated effects may occur from hy-drophilic solvent imbibition in ser-vice. Coatings on the interior cargospaces of tankers handling methanolhave been known to develop severeblisters, but only after the tankswere discharged and refilled withwater. This blistering was not seenwhere the same coatings were con-tinuously exposed to either methanolor water alone.15 Methanol uptakeby most coating films is likely. (Themolecule is small and is widely usedfor just this purpose in paint re-movers.) The retention of water-mis-cible solvent (e.g., methanol) withinthe film after the tanks are emptiedand then refilled with water will pullwater more readily into the film, os-motically producing the observedblistering in a manner similar to thatnoted above by Funke.12

In some cases, solvent-induced os-motic blistering may be quite unex-pected. It has become common forepoxy formulators in these days oflow VOC coatings to extend potlives of amine-cured epoxy systemswith ketones. Ketone solvents formlatent ketimines with amine curingagents, which effectively tie up theamine until after the coating is ap-plied. Upon application, water fromthe atmosphere reverses the reac-tion, releasing the amine as the ke-tone evaporates. The rate of dissoci-ation will probably depend upon thetype of ketone used, the relative hu-midity and temperature of applica-tion, and other factors such as pig-mentation. Platey metallic pigments,which reduce the rate of moistureingress into the wet film and ketonerelease out of the film, will prolongthe reaction in the lower layers ofthe film. So too will high film thick-nesses, which may also cause het-

erogeneous cure with the upper sur-faces curing over the uncured or thelesser cured lower layers.

Very polar solvents (e.g., ketones)associate quite readily with water.Some, such as methyl ethyl ketone(often used in this type of coating),are in fact water miscible. Should in-completely cured films of this typebe placed in immersion service be-fore complete dissociation of the ke-timine (or release of the methylethyl ketone), osmotic gradients canbe set up readily. Water penetratingthe heterogeneously cured film mayrelease ketone in the lower layersand associate with that ketone, pro-ducing osmotic blisters. While stillrare, the phenomenon is seen moreoften with coatings developed sincethe early 1980s, when ketiminecross-linking agents became morepopular. The phenomenon has beendescribed by Tator.16

Similar phenomena are also possi-ble with condensation cures inwhich alcohols are released. Oneexample is incompletely cured ethylsilicate zincs after recoating and ini-tiation of immersion service. Here,however, other failure mechanismsmay predominate, such as purestress effects leading to later splittingof the zinc film.

Corrosion from Osmotic BlisteringCorrosion in the local environmentbeneath an osmotically formed blis-ter does not necessarily occur imme-diately, especially if the liquid withinthe blister does not contain depassi-vating salts. Eventually, when corro-sion does initiate, the underside ofthe dome of the blister becomescovered in a greenish-black corro-sion product, which may itself haveosmotic consequences. As Funke13

notes, corrosion is, however, a se-quential process unconnected withinitial blister formation. In this case,corrosion of the metal beneath the

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blister probably depends upon oxy-gen permeability of the coating. Cor-rosion proceeds as shown in Fig. 5.

If the blistering phenomenon re-sults from soluble inhibitive moieties(chromates and borates) includedwithin the paint film as pigments,then the metal beneath the blistermay remain bright without formingcorrosion products for several

months. This condition can occureven under accelerated high humidi-ty test conditions designed to accel-erate blistering failure.

Once the blister is formed, corro-sion will occur by general cell activi-ty between localized cathodes andanodes on the metal beneath theblister. It will be largely controlledby oxygen permeability through the

film. Where oxygen permeability isnegligible, corrosion may be delayedindefinitely. The onset of corrosionmay also be delayed if the externalenvironment is oxygen deficient.

Given the transmission of oxygenand water to the interface, osmoticblistering may well occur in the ab-sence of externally derived salts orother hydrophilic materials, as a re-sult of the corrosion process. Thisscenario requires the pre-existenceof some site of localized deadhesionwhere water may first accumulate.Corrosion rates would initially below because of the high resistanceinhibition provided by the non-ionicwater solution. However, the forma-tion of soluble corrosion productswithin the blister would set up os-motic gradients under favorable con-ditions, leading to increased osmoti-cally induced blistering.

Thus, osmotic blistering in deoxy-genated, deionized water systems,such as are used in the nuclearpower industry, does not producecorrosion within the blister. In nu-clear power generation facilities, thevapor phase of the taurus (the cool-ing water vessel beneath the primarycontainment areas) is flooded withnitrogen gas. The nitrogen isthought to maintain bright, uncor-roded steel beneath blisters that mayform under the coating in immersedareas. In this case, the blister growthis stabilized (as the osmotic pressureinvolved is balanced by the hydro-static pressure of the head of water).

It may be prudent to ignore theblistering and leave the system inplace without repair. In Japan wherethe taurus is never drained, coatingshave provided good service for 18years or more in spite of such blis-tering. In the U.S., where similarvessels are drained for cleaning andinspection every 2 years (exposingthe interior of the blister to oxygenduring downtime), the blisters revealunderfilm corrosion. This, together

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continued


with associated cracking and dead-hesive propagation, reduces the ser-vice life of the coatings to approxi-mately 8 years.17 (Absorbed waterwill plasticize the film under wetconditions, and the wet, distensiblefilm may easily retain the blister de-formation. As the water is desorbedfrom the coating during downtime,the film becomes less plastic. Hygro-scopic tensile stresses arising fromthe drying process may exceed thetensile strength of the film, resultingin a cracking failure.)

In more usual circumstances, corro-sion will follow the onset of osmoticblistering more rapidly, although the 2phenomena remain sequential ratherthan mechanistically related. As notedabove, the rate of corrosion dependson the rate of oxygen permeabilitythrough the film and into the blister.Oxygen is consumed at the cathode

site within the blister in the formationof hydroxyl ions and cathodic depolar-ization. It will also react with the ini-tially produced ferrous ions to oxidizethem to the ferric state. This latter reac-tion soon predominates. Ferrous ionsaccumulate within the blister, consum-

ing the available oxygen immediatelyas it enters the blister cavity. The ferricions coat the underside of the blisterdome with a precipitated layer ofgreenish-black corrosion product(probably ferroso-ferric hydroxidesand magnetite, Fe304). This layer fur-

ther restricts access of oxygen to theinterior of the actual blister cavity, de-priving the underside metal of fuel forthe cathode reaction. This area (be-neath the blister dome) thus becomesuniformly polarized anodically. Thecathode sites shift to the periphery ofthe blister where the film is intact butin contact with water (laterally fromthe blister) and oxygen. Oxygen gainsaccess through the film, which is stilladherent and without the seal of cor-rosion product. The formation of ca-thodic hydroxyls at the periphery ofthe blister creates an alkaline conditionunder which adhesion may be lost;thus, for certain coatings, the area ofthe blister expands. The diffusion offerrous ions to the newly formed ca-thodic sites is followed by their precip-itation onto the cathodic steel aroundthe periphery of the blister as ferriccompounds. If at this point the film isstripped from the metal, a pattern ofannular rings of greenish-black rust isnoted, while the area of the steel im-mediately beneath the blister dome isbrighter (Figs. 1 and 6). The under-sides of the blister dome are similarlygreenish-black in color (Fig. 7).

Osmotic Blistering at PinholesOsmotic blistering is also possible atdiscontinuities in the film, as long asthe defect area is not too large (pin-holes and pores rather than abrasions,gouges, and large holidays). In thiscase, however, blistering and corro-sion phenomena are more interde-pendent. The condition is again de-scribed by Funke.13 He notes that inall cases the onset of blistering is pre-ceded by the appearance of corrosionat the pinhole, which eventually be-comes the peak of the blister dome.The defective, semi-transparent mem-brane caused by the blister is repairedby a plug of corrosion product (hy-drated iron oxide), which forms at thebottom of the pinhole channel. Underthe sealed conditions, blister forma-tion by osmosis can now occur. The

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continued

Fig. 6 - Photomicrograph of steel surface beneath osmotically blistered coating. Note steel surface beneath dome of blister is bright and surrounded by circular areas of green-black corrosion product.

Fig. 7 - Underfilm condition after osmotic blister formation—showing the deposit of corrosionproduct underneath blister domes (the other side of the interface shown in Fig. 6, from whichfilm delaminated)


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soluble species is soluble ferrouscompounds. Corrosion products be-neath the plug sites set up an osmoticgradient across the now repaired film.Again, the growing blister becomesoxygen depleted because the ferrousto ferric oxidation process consumesthe available oxygen as soon as theoxygen enters the blister. Anodic po-larization of the base of the blister siteand the shift in cathodes to the blisterperipheries follow as noted above.

ConclusionNext month’s article will reviewnon-osmotically driven blistering ofcoatings on metal. ❒

References1. L.A. van der Meer-Lerk and P.M.

Heertjes, “Blistering of VarnishFilms on Substrates Induced bySalts,” JOCCA (March 1979), 79.

2. L. Igetoft, “Relation between Sur-face Cleanliness and Durabilityof Anticorrosive Paints on Struc-tural Steel,” in Proceedings of2nd World Bridge Conference,New York, NY, October 26-27,1982 (St. Louis, MO: Universityof Missouri-Rolla, 1983), p. 5.

3. J. West, “The Relationship Be-tween Coating Thickness andSalt Contamination on Blisteringof Coatings,” UK Corrosion/85Harrogate, November 4-6, 1985.

4. M. Morcillo, S. Feliu, J.C. Galvan,and J.M. Bastidas, “Some Obser-vations on Painting Contaminat-ed Rusty Steel,” JPCL (September1987), 38.

5. B.R. Alblas and A.M. van Lon-den, “The Effects of ChlorideContamination on the Corrosionof Steel Surfaces: A Literature Re-view,” Protective Coatings Eu-rope (February 1997), 18.

6. M. Morcillo, S. Hernandez, J.Simancas, S. Feliu Jr., and S.Gimenez, “Underfilm Corrosion ofSteel Induced by Saline Contami-nants at the Metal/Paint Inter-face,” JOCCA (January 1990), 24.

7. W.C. Johnson, “Detrimental Ma-terials at the Steel/Paint Inter-face,” New Concepts for CoatingProtection of Steel Structures,ASTM STP 841, eds. D.M. Bergerand R.F. Wint (Washington, DC:ASTM, 1984), p. 28.

8. B.R. Appleman, S.K. Boocock, R.E.Weaver, and G. Soltz, Effect ofSurface Contamination on CoatingLife, SSPC Report 91-07 (FHWAReport RD-91-011) (Pittsburgh,PA: SSPC, June 1991).

9. H. Gross, “Examination of SaltDeposits Found under GermanPainted Bridges,” Materials Per-formance (October 1983), 28.

10. W. Wettach and the Pittsburgh So-ciety for Paint Technology, “AStudy of Factors Affecting the Rust-ing of Steel and Blistering of Or-ganic Metal Coatings—II,” OfficialDigest (November 1961), 1427.

11. T.R. Bullet and J.L. Prosser, “Cor-respondence on Swelling and

Blistering in Paint Films,” JOCCA(December 1962), 836.

12. A.C. Elm, “Zinc Dust Metal Pro-tective Coatings,” New JerseyZinc Co. Publication, New York,NY, May 1968.

13. W. Funke, “Blistering of PaintFilms & Filiform Corrosion,”Progress in Organic Coatings,Vol. 9, p. 29.

14. W. Funke, “Preparation andProperties of Paint Film with Spe-cial Morphological Structure,”JOCCA (November 1976), 398.

15. Private Communication, G. Tin-klenberg, 1996.

16. K.B. Tator, “Can Failures StillOccur When the Correct Coating(for a Given Environment) Is Se-lected and Applied Properly?” inCorrosion Control by OrganicCoatings, ed. H. Leidheiser (Hous-ton, TX: NACE, 1981), p. 122.

17. Private Communication, S.J.Oechsle, 1996.

Documents

Trouble With Paint Blistering of Paint Films on Metal Part 1 Osmotic Blistering